9702 · 19.2
Energy stored in a capacitor flashcards
Revision flashcards for Cambridge 9702 Energy stored in a capacitor (syllabus 19.2). Flip, recall, then mark a real past-paper question.
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In what form is energy primarily stored in a capacitor?
Electrical potential energy.
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How can the energy stored in a capacitor be determined graphically?
By finding the area under its charge-voltage (Q-V) graph.
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State two common formulas used to calculate the energy stored in a capacitor.
$E = \frac{1}{2}QV$ and $E = \frac{1}{2}CV^2$.
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Describe the pattern of change for charge and voltage in a discharging capacitor.
Both charge and voltage decrease exponentially over time.
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What does the time constant ($\tau$) of an RC circuit indicate during discharge?
The time taken for the charge, voltage, or current to fall to approximately 37% (or $1/e$) of its initial value.
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What is the fundamental relationship defining capacitance (C)?
$C = \frac{Q}{V}$, where Q is charge and V is potential difference.
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What is the unit of capacitance and what does it represent?
Farad (F). 1 Farad = 1 Coulomb of charge stored per Volt.
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What is the shape of a capacitor's Q-V graph?
A straight line passing through the origin.
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How is the third energy formula, $E = \frac{1}{2}\frac{Q^2}{C}$, derived?
By substituting $V = Q/C$ into $E = \frac{1}{2}QV$.
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What is the formula for the time constant ($\tau$) in an RC circuit?
$\tau = RC$.
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What happens to Q, V, and I during capacitor discharge after one time constant ($\tau$)?
They decrease to about 37% of their initial value.
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Write the exponential decay equation for charge during discharge.
$Q = Q_0 e^{-t/\tau}$.